The present invention relates to a novel method of preparing biodegradable aliphatic polyesters which can be produced by polycondensation between aliphatic diols and aliphatic diacids or esters thereof.
It is known that aliphatic polyesters represent a class of biodegradable polymers. The feasibility of using them to produce products, however, is linked to the achievement of high molecular weights which enable the products to have suitable qualities such as, for example, adequate mechanical strength, which render them fit for use.
In the literature, various methods of achieving this objective have been tried.
In U.S. Pat. No. 5,306,787 and in European patent application EP-0 565 235, the use of diisocyanates as chain extenders on the fused polymer is described. In patent application EP 0 747 416 the use of triisocyanates alongside diisocyanates is also reported.
The use of these chain extenders makes the polymerization process more complex since it requires two distinct steps: the actual polymerization step, and the upgrading step to produce high molecular weights. In these cases, the polymerization process is also dangerous to health and capable of unfavourably modifying the biodegradability of the materials and giving rise to potentially toxic substances during the biodegradation processes.
According to U.S. Pat. No. 5,741,882, the production of saturated polyesters of high molecular weight from diesters and glycols is linked basically with the particular way in which the catalytic system is added, and with the presence of a scavenger of free radicals. The catalyst, which is constituted by alcoholates or acetyl acetonates of various transition metals, preferably of titanium since they are indicated as being most active, is added at at least two moments in the course of the polymerization step, preferably at the beginning and during the deglycolation.
In addition to the complex and ill-defined way of adding the catalyst, the method claimed suffers from some important limitations such as the need to start with esters of the saturated acids, and the fact that at least 30% of the esters are succinates and that at least 70% of the glycols comprise 1,4-butandiol.
Limitations in structure are also present in the method described in the patent WO94/14870. The dicarboxylic acid used is in fact succinic acid or a mixture thereof with another aliphatic diacid. The method also provides for a particularly complex catalytic system comprising a catalyst for the first stage of the polymerization and a catalyst for the second stage. In fact, during the esterification or transesterification step, the use of tetrabutyl titanate, alone or also mixed with calcium or zinc acetates, tetrapropyl titanate, or dibutyl tin oxide, is provided for. During the deglycolation stage, dibutyl tin oxide is used, alone or mixed with butyl, iso-propyl, or n-propyl titanates or calcium acetate.
Moreover, the polymerization in solvent described in patent application EP-0 618 249 appears not to be easy to implement in practice owing to the need to use and to dehydrate large quantities of high-boiling solvents such as diphenyl ether. There is a further difficulty when the solubility of the polymer is such as to require large quantities of precipitating solvent for the isolation.
With regard to the use of catalysts in the polyester preparation process, various classes of inorganic or organometallic tin compounds, used mainly for the production of aromatic polyesters in the polyesterification or transesterification step, have been described (U.S. Pat. Nos. 4,970,288, 5,166,310). In no case, however, is the use of these compounds described for the preparation of biodegradable aliphatic polyesters.
Moreover, amongst organometallic tin compounds, butylstannoic acid is not described as having a greater activity for the production of biodegradable aliphatic polyesters than the other tin derivatives.